Wastewater treatment is a process for turning wastewater into water than can either be disposed of by returning it to the water cycle or reused as potable water. If the wastewater is primarily from municipal sources, or sewage, the process is called sewage treatment.
Most sewage treatment facilities have at least four treatment stages. In the pretreatment stage, large objects are removed from sewage using a bar screen. Large objects include tree branches, leaves, limbs, trash, and other non-biological sources of waste.
After large objects are removed via pre-treatment, the sewage will be subjected to primary treatment. In this stage, the sewage is temporarily held in pre-settling basins where heavy solids can settle to the bottom while lighter materials can float to the surface. The floating materials are removed and the level of liquid is regulated, discharging when needed due to rain.
Secondary treatment uses microorganisms to degrade the chemical and biological components of the sewage, including human waste, food waste, soaps, and detergents. Microorganisms, such as bacteria and protozoa will consume and break down much of the chemical and biological components dissolved or suspended in sewage. After secondary treatment, at least two materials are still left: water and sludge.
A fourth stage in sewage treatment is to treat the remaining water and sludge. After the biological secondary treatment, sludge separates from water by gravity. The remaining water can then be further purified, depending on its ultimate use. The wastewater can be treated to remove phosphorus, nitrogen and other nutrients, or disinfected with chlorine, ozone or ultraviolet.
Sludge can be composed of water and a variety of accumulated solids from the previous treatment stages. While the treated water has a clear destination, either a return to the water cycle or into the drinking water supply, there are fewer options for the sludge. Moreover, the sludge also traps large quantities of water. The water remaining in sludge can be hard to remove and reusing it can be energy intensive compared with using the water that easily separates from the sludge using gravity as described above in previous treatment stages.
Sludge can either be recycled or disposed in landfills. Two strategies can be employed to process the solid components in sludge for recycling: (1) energy recovery through generation of methane or (2) use as fertilizer. First, some energy can be recovered through anaerobic digestion of sludge, which produces methane. Methane can then be burned to partially recover the energy cost of processing sludge. Another possible destination for sludge is to be used as a fertilizer. However, for sludge to be used for energy recovery or as fertilizer, the sludge must either be transported offsite for processing and/or dried for adequate use. Sludge that is not processed ends up in landfills, but is generally dried prior to placement to lower refuse costs. When transporting sludge, the more water, the less concentrated the sludge, which leads to higher transportation costs. The sludge can be concentrated through centrifugation or conventional drying.
EPA 503 governs the use or disposal of sludge EPA has put forth a comprehensive set of rules and guidelines for the handling of these sewage solids and the need to render it free from pathogens prior to disposal or use. As a result, communities are reluctant to accept the material as a soil additive due to the possibility for changing regulations on the disposal of waste sludge. This puts a higher burden on each plant to bring their processes into line to meet the EPA 503 guidelines. Currently, the equipment most widely used is a belt press which removes 20-30% of the liquid in the solids with the addition of additives such as lime that helps bind the solids. Using the additives adds additional costs and weight to the transportation expense but does nothing to address the reduction of the pathogen found in the sludge. As a result, the cost of transportation from the plant to the disposal site continues to rise.